1. Technical Field
The present invention relates to fuel control systems for stationary and propulsion gas turbine engines.
2. Description of the Related Art
The high burn rates of gas turbine engines require the fuel delivery systems to be capable of rapidly and precisely metering fuel. Traditionally, fuel delivery systems for turbine engines, particularly those used for jet propulsion, have included a fuel pump, a pressure accumulator and a fuel metering device, all of which are separate components mounted on or near the engine and coupled to the engine and fuel source by suitable fuel lines. The accumulator operates to dampen pulsation or ripple in the fuel caused by the pump so that the metering device can accurately dispense the appropriate amount of fuel to the engine fuel atomizer. The use of multiple components is expensive and occupies space, which is especially limited for propulsion systems.
It is desirable to reduce the number of components in the fuel delivery system by combining the fuel pump and metering device into one unit. However, such combined devices must meet both the extreme pump and the metering requirements for turbine engines. Specifically, it must be able to pump particle contaminated fuel for an extended time period. It must have good dry lift capability and be able to operate with high vapor-to-liquid ratios at the pump inlet. Moreover, if no accumulator or fluid muffler is to be used, the pump must also be able to provide generally non-pulsating fuel flow. It should be exhibit low power consumption and hysteresis and operate with high efficiency and low friction. The device must also have a high turn-down ratio to accurately meter a wide range of flow rates. Additionally, the device must be compact and have minimal internal leakage.
In the turbine industry, the fuel delivery systems typically employ gear pumps which create a pressure differential by moving the fuel through a series of intermeshing teeth running at a high frequency. Gear pumps consume a lot of power and leak internally and are therefore less than ideal for jet engine use. Moreover, due to reliability concerns, gear pumps used for propulsion applications typically are powered by an engine driven gear box (rather than an electric motor) and therefore must be coupled to a separate metering valve via suitable fuel lines, which increases expense and occupies additional space.
U.S. Pat. No. 6,371,740, assigned to the assignee of the present invention and hereby incorporated as though fully set forth herein, discloses a fuel metering pump for turbine engines. The metering pump employs a rotating face cam to alternately reciprocate a pair of actuators that in turn drive a pair of rolling diaphragms to pump and meter the fuel. The metering pump is specially designed to drive the pumping members at a constant velocity to minimize pressure ripple and thus provide essentially non-pulsed metering of the fuel. The rolling diaphragm design assists in keeping contaminants commonly found in jet fuel from degrading the working components of the metering pump.
While the aforesaid metering pump provides a marked improvement in accurate fuel metering at high flow rates, the diaphragms have pressure limitations that can make it less suitable for certain sustained high pressure applications. In particular, it can be necessary in some jet engine applications to achieve a sustained pressure rise of over 500 psi. This pressure must be generated and maintained while metering the high flow rates required for sustained combustion, which can be 700 pph or more.
U.S. application Ser. No. 10/891,269, filed Jul. 14, 2004, assigned to the assignee of the present invention and hereby incorporated as though fully set forth herein, discloses a precision fuel metering pump suited for the aggravated temperature and pressure conditions of turbine engines applications having a unique cam-driven double-ended spool piston arrangement that is very efficient and accurate with little leakage and wide operational parameters. The pump is contained in a compact package, however, a separate DC motor is mounted to the pump housing to drive the piston cam and piston arrangement. Moreover, other components used to control fuel flow must be coupled to this device to achieve proper fuel control, for example, the motor drive control, a fuel shut off valve and a flow divider, which divides flow between the primary outlet port and a secondary outlet port to send fuel to the secondary burner nozzle(s) of the engine after engine light off. These additional components may require their own housings as well as conduit and other fluid and electrical line connections, thus increasing the space and weight requirements of the system. Further, the use of several separate components makes installation and replacement of the control system more time consuming and costly.
Moreover, this pump relies on small, open weave inlet filters to filter debris and contaminants from the fuel prior to entering the pump. Open weave type filters are used to minimize the pressure drop across the inlet, while still providing some filtering, so that the pump will operate at low inlet pressure, near true vapor pressure, without cavitation. However, this comes with somewhat reduced filtering capacity. Excessive debris in the pump can cause binding of the operating member, and reduce its efficiency or lead to pump failure. Thus, filtering of the fuel can be critical to proper performance of the pump and the fuel control system generally.